The present invention relates to an apparatus for the detection and signalling of an unauthorized opening and/or fracture or other positional shift of a movable partition, such as a window or sliding door, which apparatus is substantially tamper-proof, easy to install and not susceptible to accidental damage.
Prior systems, heretofore used to detect the unauthorized opening of doors and windows, and known to me, have utilized mechanically or magnetically operated switches mounted on the door or on the window. In one such prior art structure, a magnetically operated reed switch whose contacts are spring-biased to open position is affixed to the window sash, i.e. to the movable part of the window to be guarded, and a magnet is mounted on the frame of the window opposite the switch. As the sash moves to open the window, the switch moves out of the field of influence of the magnet. The switch contacts thereupon open and this activates a burglar alarm or other signalling device arranged to be activated upon opening of the circuit in which the reed switch is included.
In a second prior art system, a mechanically operated switch having normally open pushbutton or cam-operated contacts is mounted on the window sash. The switch is operated by a wiper arm or cam affixed to the window frame such that, when the window is closed, the contacts are closed. As the window is opened, the switch moves out of engagement with the wiper arm, its contacts open, and cause the alarm to be activated.
It has also been proposed to reverse this arrangement of parts, by putting the switch on the window frame while putting the actuating element therefor, whether a magnet or a cam, or of other nature, on the sash in such position that this element holds the switch in the condition (e.g. a closed contact condition) for which the alarm is not activated, when the window is closed.
In all of these prior art constructions, either the switch or the actuating element is mounted on the sash and the other is mounted on the window frame immediately adjacent thereto, such that both the switch and the switch actuating element can be easily seen by a potential intruder looking in through the window. Once seen, the prior art devices can be fairly easily deactivated by reaching in through a hole cut in the window pane and attaching a magnet to the reed switch, by taping the pushbutton of a mechanical switch in the closed position, or by bypassing the switch by means of jump wires.
Furthermore, with the switch element (i.e. the switch proper) and the actuating element therefor, one on the frame and the other on the sash, there is a risk of false alarms due to lateral motion of the sash under the influence of the wind, for example, even when no effort is made to move the sash up or down.
These considerations also apply to alarm systems of the prior art types hereinabove described as mounted on or employed in conjunction with casement windows and other movable partitions.
Another defect of the prior art devices is that since the switches are mounted on the window sash or frame, they are exposed to the weather when the windows are open and are thereby subject to deterioration.
It has also been proposed in the prior art to apply to the glass pane of a door or a window a strip of conductive foil which is ruptured by the breakage of the glass thereby activating a burglar alarm connected to the strip. In order to permit movement of the window, the prior art systems utilized a flexible cord to connect the conductive strip on the pane with the burglar alarm circuit. This cord was unsightly or inconvenient if long enough to permit substantial motion of the pane, or else it required for the purpose manual plugging and unplugging of a connector to join the cord to the remainder of the circuit at the jamb.